The proposed experiments are designed to elucidate mechanisms by which recessive mutations become expressed in near-diploid cultured mammalian cells. We have previously observed in mouse CAK cells that approximately one-quarter to one-half of the recessive mutations at two autosomal loci [adenosine kinase (Adk) and adenine phosphoribosyl transferase (Aprt)] are associated with chromosome segregation. Apparently a single mutational hit has occurred in each of these clones, and the chronmosome carrying the other, wild-type allele has been lost through non-disjunction; in some cases the chromosome carrying the mutant allele has duplicated. Experiments are proposed to determine what other secondary genetic events could be involved in the expression of recessives in cells that have retained the original two copies of the relevant chromosome. We will use a clone of CAK cells heterozygous for a physical marker on chromosome 8 to ask whether mitotic recombination or gene conversion can ever bring about the expression of Aprt mutations. A cloned Chinese hamster Aprt gene will be used as a molecular probe to find out whether the two mutant alleles on the different homologues are identical. The two chromosomes will be studied individually in mouse x hamster hybrids. Similar experiments will be done to find out whether deletion of the gene from one chromoxome is ever responsible for expression of mutant allele on the other homologue. Other experiments will be done to search directly for the occurrence of miototic recombination or gene conversion in mouse cells. These studies will involve the introduction of DNA sequences into cells by transformation. Cells will be transformed with a plasmid carrying one copy of a DNA sequence that is present in about 30,000 copies in mouse cells. There are two classes of sequence in this DNA family, as defined by restriction enzyme mapping. We will ask whether the transforming sequence ever becomes converted to a sequence of the other class. A study of Aprt mutants of human lymphoblastoid cells will be done, to find out whether mechanisms for the expression of recessives are similar to those in mouse fibroblasts. Finally, cells heterozygous at the Aprt locus will be used to ask whether tumor promoters induce mitotic aneuploidy in mammalian cells.